To some degree I think it's more the nature of Internet style communication (text / typing oriented with no face to face interaction) in itself. Facebook after all is not anonymous and is not 100% free of this sort of thing.

Honestly, though, there are some corners of real life (especially in those early teen years as you allude to, but even among full grown adults that should know better) where you will find people with this sort of personality. These type of reactions are not surprising in that context. Social media does seem to do something to amplify raw vulgarity and classless behavior though.